1. Field of the Invention
This invention relates to an aqueous coating composition for metal sheets, and surface-treated metal sheets and a surface treating method using the same. More detailedly, this invention relates to an aqueous coating composition capable of forming a film giving good adhesion of UV-curable paints, UV-curable inks or UV-curable adhesives, corrosion resistance, formability, and durability with injury when applied onto the surface of a metal sheet, and surface-treated metal sheets and surface treating method for metal sheets using the same.
2. Statement of Related Art
Workpieces and formed products using metal sheets, for example aluminum or aluminum alloy sheets have hithereto been utilized in broad fields, and there are many techniques for forming resin film on their surfaces in order to give formability, corrosion resistance, durability with injury, etc. For example, JP-A-8/127735 discloses an aluminum sheet for lubricating treatment which comprises an aluminum sheet having chromate film provided thereon and having lubricity film composed of a water dispersion-type polyurethane resin, particles of a silicon compound and a lubricant formed on the chromate film, JP-A-7/228828 discloses a resin-coated aluminum alloy material excellent in lubricity and excellent in corrosion resistance which makes press forming of strong processing possible, produced by forming resin film using a composition wherein particles of a polyethylene wax are dispersed in a resin. Further, JP-A-7/195031 discloses, as a surface-treated aluminum sheet material capable of having excellent formability and durability with injury, a surface-treated aluminum sheet material having a friction coefficient controlled to be 0.15 or less, which comprises an aluminum alloy sheet material having a coating containing 5 to 40% SiO2 having a particle size of 0.1 xcexcm or less and 5 to 60% a lubricant applied thereon with a thickness of 0.5 to 10 xcexcm.
All these metal sheets of prior art are materials having good formability, corrosion resistance, etc. given by forming resin film containing a lubricant such as wax particles, and as to painting adhesion after formation, it is mentioned that painting adhesion can be ensured by anchor effect by pinholes in JP-A-7/228828, by specifying especially the combination of the particle size of the silicon compound in the lubrication treating agent, the kind and compounding amount of the lubricant and the particle size of the resin wax in JP-A-8/127735, and by limiting the amount of film formed on the surface in JP-A-7/195031.
In recent years, from compliance with controls on solvent effluence and advantages of work, cases where as paints for top coating or inks, UV-curable ones are used have been increasing. However, in UV-curable top coating, the volumetric shrinkage rate between before and after curing is large due to radical polymerization, and as a result, the cured film has remaining large internal stress. Therefore, the film of UV-curable top coatings is inferior to the film of usual solvent-type or aqueous top coatings in adhesion to the base material, and there arise many cases where base material design taking UV-curable top coatings into account is made necessary. The surface of sheets treated by the above-exemplified prior art is often poor in adhesion of UV-curable top coatings.
As stated above, film satisfying adhesion of UV-curable top coatings has not been obtained by usual surface treating methods wherein resin film is formed for giving formability, corrosion resistance, durability with injury, etc. onto the surface of metal sheets. This invention aims to solve this problem, and provides such an aqueous coating composition appliable to the surface of metal sheets such as aluminum or aluminum alloy sheets that the coating film shows good adhesion of UV-curable top coatings, in addition to the above various characteristics, and surface-treated metal sheets and a surface treating method for metal sheets using the same.
The invention for solving the above problem relates to an aqueous coating composition for metal sheets which comprises, as indispensable components, the following (C) alone or two or more of the following (A), (B) and (C), and water, and does not contain a photopolymerization initiator:
(A) a polyurethane resin having an elongatedness of dry film of 100% or more (excluding the following (C))
(B) a synthetic resin having at least one ethylenic double bond (excluding the following (C))
(C) a polyurethane resin having an elongatedness of dry film of 100% or more and having at least one ethylenic double bond.
The invention also relates to a metal sheet having thereon film formed by applying and drying the aqueous coating composition.
The invention also relates, as a method for effectively using the above aqueous coating composition for metal sheets, to a surface treating method for a metal sheet which comprises coating the surface of a metal sheet with the aqueous coating composition for metal sheets, drying the applied composition and forming a film of a UV-curable paint, a UV-curable ink or a UV-curable adhesive on the whole or part of the resultant surface.
The invention also relates to a metal sheet having thereon composite film formed by the surface treating method.
When the composition and the surface treating method are used, good adhesion of UV-curable top coatings is obtained in addition to good formability, good corrosion resistance and good durability with injury.
When, in the composition and method, the composition contains (D) a water dispersible wax in the range of 0.1 to 20% by weight of the total solid components, good formability in strong forming is obtained in addition to the above various characteristics, and when the composition contains (E) a water insoluble fine particles having a Mohs hardness of 2.5 or more in a rate of 30% by weight or less of the total solid components, better durability with injury is obtained in addition to the above various characteristics.
The invention is detailedly described below. In the invention, (A) the polyurethane resin having an elongatedness of dry film of 100% or more is assumed to mean such a polyurethane resin that when an aqueous dispersion thereof is applied onto a support sheet, dried at room temperature for 24 hours and further dried at 120xc2x0 C. for 10 minutes, and the resultant dry film (150 xcexcm thickxc3x9710 mm wide) is stretched at 300 mm/min, elongatedness at breaking is 100% or more. When the elongatedness is less than 100%, the film is poor in following characteristic to deformation in the case where the metal sheet after the coating and drying is processed, and cracks are liable to be generated at the processed part, and thereby corrosion resistance lowers. The upper limit of the elongatedness is not particularly limited, but when it goes beyond 800%, film hardness generally gets too low and the practical range gets narrow in durability with injury, and the elongatedness is preferably 800% or less.
As the above polyurethane resin, general polyurethane resins can be used so long as the elongatedness is satisfied. Namely, the above polyurethane resin of the invention can be a polyurethane resin obtained by polyaddition reaction between a polyisocyanate having two or more (usually two) isocyanato groups and a polyol having two or more (usually two) hydroxyl groups.
As the polyisocyanate, aromatic, alicyclic and aliphatic polyisocyanates are mentioned, and, specifically, aromatic polyisocyanates include tolylene diisocyanate (TDI)(2,4- or 2,6-TDI), diphenylmethane diisocyanate (MDI)(4,4xe2x80x2- or 2,4xe2x80x2-MDI), polymeric MDI, xylylene diisocyanate (XDI), naphthylene diisocyanate (NDI)(usually 1,5-NDI), paraphenylene diisocyanate (PPDI), tetramethylxylylene diisocyanate (TMXDI), tolidine diisocyanate (TODI), 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenylene diisocyanate, etc., alicyclic polyisocyanates include dicyclohexylmethane diisocyanate (HMDI)(4,4xe2x80x2- or 2,4xe2x80x2-HMDI), isophorone diisocyanate (IPDI), isopropylidene-bis-(4-cyclohexylisocyanate)(IPC), hydrogenated xylylene diisocyanate (hydrogenated XDI), cyclohexylene diisocyanate (CHPI)(usually 1,4-CHPI), 1,5-tetrahydonaphthalene diisocyanate, etc., and the aliphatic polyisocyanates include hexamethylene diisocyanate (HDI), lysine diisocyanate (LDI), tetramethylene diisocyanate, etc.
As the polyol, there can, usually, be mentioned (poly)alkylene glycols, polyester polyols, polyols having Cxe2x80x94C bonds as the principal chain and other polyols.
The (poly)alkylene glycols include (poly)ethylene glycols (ethylene glycol, diethylene glycol, triethylene glycol, etc.), 1,2-propylene glycol, 1,3-propylene glycol, polyethylen/propylene glycol, neopentyl glycol, 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 3-methyl-1,5-pentanediol, hexamethylene glycol, etc.
The polyester polyols include polyester polyols having hydroxyl groups at the ends obtained by polycondensation between low molecular weight polyols such as (poly)alkylene glycols as mentioned above, bisphenol A, hydrogenated bisphenol A, trimethylolpropane or glycerol and polybasic acids such as succinic acid, glutaric acid, adipic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, tetrahydrophthalic acid, endomethylenetetrahydrophthalic acid or hexahydrophthalic acid.
The polyols having Cxe2x80x94C bonds as the principal chain include, for example, methyl (meth)acrylate-vinyl alcohol copolymers, polyolefins such as polyethylene or polypropylene having hydroxyl groups at both ends, partial or complete hydrolyzates of ethylene-vinyl acetate copolymers, etc.
The other polyols include, for example, bisphenol A, hydrogenated bisphenol A, trimethylolpropane, glycerol, etc.
The molecular weight of the polyurethane resin having an elongatedness of dry film of 100% or more of (A) used in the invention is preferably on the order of 500 to 1,000,000 when measured by gel permeation chromatography. As the polyurethane resin having an elongatedness of dry film of 100% or more of (A), it is also possible to use, as commercial products, HYDRAN HW-340 (water dispersion of a polyester polyol-type polyurethane resin, elongatedness 280%, made by DAINIPPON INK AND CHEMICALS, INC.), HYDRAN HW-312B (water dispersion of a polyalkylene glycol-type polyurethane resin, elongatedness 700%, made by DAINIPPON INK AND CHEMICALS, INC.), HYDRAN HW-950 (water dispersion of a polyester polyol-type polyurethane resin, elongatedness 120%, made by DAINIPPON INK AND CHEMICALS, INC.), PERMARIN UA-110 (water dispersion of a polyalkylene glycolxe2x80x94type polyurethane resin, elongatedness 500%, made by SANYO CHEMICAL INDUSTRIES, LTD.), PERMARIN UA-310 (water dispersion of a polyester polyol-type polyurethane resin, elongation 420%, made by SANYO CHEMICAL INDUSTRIES, LTD.), etc.
As the synthetic resin having at least one ethylenic double bond of (B) used in the invention, there can be mentioned synthetic resins such as epoxy-series resins, polyester-series resins or polyurethane-series resins having at least one, preferably two or more ethylenic double bonds, and diallyl phthalate-series resins.
As the epoxy-series resins having at least one ethylenic double bonds, there can be mentioned a bisphenol-type epoxy resin-series ethylenic double bond-containing resin obtained by adding an ethylenic double bond-containing monocarboxylic acid (e.g., acrylic acid, methacrylic acid, crotonic acid, etc.) to a bisphenol-type epoxy resin having epoxy groups at both ends formed through the reaction between the epoxy groups and the carboxyl groups, the epoxy resin being formed by polycondensation and polyaddition between bisphenol A, bisphenol F or hydrogenated bisphenol A or a mixture at any combination of bisphenol A, bisphenol F and hydrogenated bisphenol A and epichlorohydrin. Typical examples thereof include bisphenol A-type epoxy resin-series ethylenic double bond-containing resins represented by the following general formula obtained by using bisphenol A and (meth)acrylic acid in the above. 
(wherein R represents a hydrogen atom or a methyl group and n represents a polymerization degree)
The molecular weight of the above bisphenol-type epoxy resin-series ethylenic double bond-containing resins is preferably on the order of 600 to 1,000,000 when measured by gel permeation chromatography.
As the polyester-series resins having at least one ethylenic double bond, there can be used usual unsaturated polyester resins obtained by polycondensation between unsaturated dicarboxylic acids or anhydrides thereof (maleic acid, fumaric acid, itaconic acid, maleic anhydride, fumaric anhydride, etc.) and glycols (ethylene glycol, propylene glycol, 1,3-butylene glycol, 1,6-hexanediol, di- or triethylene glycol, neopentyl glycol, hydrogenated bisphenol A, etc.). The unsaturated polyester resins can also be ones wherein part of the above unsaturated dicarboxylic acids are replaced by phthalic acid, isophthalic acid, succinic acid or the like.
As the polyester-series resins having at least one ethylenic double bond, there can also be used resins obtained by polyaddition and polycondensation or merely polyaddition between glycidyl (meth)acrylate and alkadienoic acids (succinic acid, adipic acid, etc.) or anhydrides thereof or alkedienoic acids (maleic acid, fumaric acid, etc.) or anhydrides thereof through the glycidyl groups and the carboxyl groups.
As the polyester-series resins having at least one ethylenic double bond, there can also be used methacrylic acid dimer-polyol copolymers represented by the following general formula. 
(wherein m represents an integer of 1 to 3 and n represents a polymerization degree)
As the polyester-series resins having at least one ethylenic double bond, there can also be used 2,3-dimethyl-1,3-butadiene-modified bisphenol A propoxide fumarate polyester resins represented by the following general formula. 
(wherein A represents 
and n represents a polymerization degree)
As the polyester-series resins having at least one ethylenic double bond, there can also be used a resin obtaind by esterifying with (meth)acrylic acid at least one end of a copolymer having OH groups at both ends obtained by polycondensation or polyaddition and polycondensation between phthalic acid or isophthalic acid or an anhydride thereof and a glycol (ethylene glycol, propylene glycol, etc.).
As the polyester-series resins having at least one ethylenic double bond, there can also be used a resin obtaind by reaction between the above-mentioned unsaturated polyester resin and glycidyl (meth)acrylate through the end OH group or end carboxyl group of the unsaturated polyester resin and the glycidyl group.
The molecular weight of the polyester-series resins having at least one ethylenic double bond as mentioned above is preferably on the order of 1,000 to 100,000 when measured by gel permeation chromatography.
As the polyurethane-series resin having at least one ethylenic double bond, there can generally be used a resin obtaind by reacting a polyurethane resin having NCO groups at both ends obtained by polyaddition reaction between a polyisocyanate having two or more (usually two) of isocyanato groups and a polyol having two or more (usually two) of active hydrogen atoms with a compound having an ethylenic double bond and an OH group together in the molecule between the NCO group and the OH group.
As the polyisocyanate and the polyol, there can be used polyisocyanates and polyols, respectively, mentioned in the description of the polyurethane resin having an elongatedness of dry film of 100% or more of the above (A).
As the compound having an ethylenic double bond and an OH group together in the molecule, there can be mentioned 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl(meth)acrylate, N-methylol(meth)acrylamide, 2-hydroxyethyl-o-xylylene(meth)acrylate, 3-hydroxypropyl-o-xylylene(meth)acrylate, trimethylolpropane diallyl ether, etc.
As the polyurethane-series resin having at least one ethylenic double bond, there can also be used a resin represented by the following general formula obtained by urethanizing hydroxyethylphthaloyl methacrylate with xylylene diisocyanate. 
(wherein n represents a polymerization degree)
As the polyurethane-series resin having at least one ethylenic double bond, there can also be used a resin represented by the following general formula obtained by urethanizing trimethylolpropane diallyl ether with tolylene-2,4-diisocyanate. 
The molecular weight of the polyurethane-series resins having at least one ethylenic double bond as mentioned above is preferably on the order of 600 to 1,000,000 when measured by gel permeation chromatography.
As the synthetic resin having at least one ethylenic double bond of (B), there can also be used polyvinyl alcohol-series resins, polyamide-series resins, poly(meth)acrylic acid-series resins, (meth)acrylic acid-maleic acid copolymer-series resins, silicone resins, etc. having at least one ethylenic double bond.
As the diallyl phthalate-series resin as one of the synthetic resins having at least one ethylenic double bond of (B), there can also be used a homopolymer of diallyl phthalate or diallyl isophthalate or a copolymer of both. The molecular weight of the diallyl phthalate-series resin is preferably on the order of 2,000 to 1,000,000 when measured by gel permeation chromatography. As the polyurethane resin having an elongatedness of dry film of 100% or more and having at least one ethylenic double bond of (C), there can be used, among polyurethane-series resins having at least one ethylenic double bond mentioned in the description of the above (B), those having an elongatedness of dry film of 100% or more.
As commercial products of (C), there can be used R-5000 (water dispersion of a polyurethane acrylate, elongatedness 340%, made by DAI-ICHI KOGYO SEIYAKU CO., LTD.), etc.
As to the mutual ratio of the resins (A), (B) and (C), it is necessary that (A)+(C) is 50 to 100% by weight, and (B)+(C) is 0.1 to 100% by weight based on the resins (A)+(B)+(C). When (A)+(C) is less than 50% by weight, the slippage and abrasion resistance of the film surface necessitated at the time of forming get insufficient, and when (B)+(C) is less than 0.1% by weight, the adhesion of UV-curable paints, inks or adhesives gets insufficient. (A)+(C) is preferably 60 to 100% by weight, and (B)+(C) is preferably 5 to 100% by weight.
As to the total concentration of the resins (A), (B) and (C) in the aqueous coating composition for metal sheets of the invention, there is no particular limitation, but when it is too low, it gets difficult to form necessary film when applied, and when it is too high, it gets difficult to maintain the composition as water dispersion, and the proper total concentration is on the order of 5 to 70% by weight, particularly on the order of 10 to 60% by weight.
The aqueous coating composition for metal sheets of the invention does not contain any photopolymerization initiator. In the invention, the photopolymerization initiator is assumed to mean a substance forming a radical polymerization-activating substance or an ionic polymerization-activating substance initiating polymerization or crosslinking of monomer(s) having an ethylenic double bond by exposure to light, particularly ultraviolet rays. UV-curable paints, inks or adhesives contain a photopolymerization initiator at least at the time of use, and the aim of the invention is to ensure adhesion between the films by making the double bonds in the undercoat film formed from the composition of the invention participate at the time of chain polymerization initiated by the photopolymerization initiator.
Film obtained by applying onto a metal sheet the aqueous coating composition for metal sheets of the invention containing the resin (C) alone or two or more of the resins (A), (B) and (C) and not containing a photopolymerization initiator shows good adhesion of UV-curable top coatings, in addition to good formability, good corrosion resistance and good durability with injury.
When the aqueous coating composition for metal sheets of the invention containing the resin (C) alone or two or more of the resins (A), (B) and (C) further contains (D) a water dispersible wax in a range of 0.1 to 20% by weight of the total solid components, the formed film shows good lubricity even at the time of strong forming such as bending, drawing or punching of metal sheets, in addition to goodness of the above various characteristics, and gives good formability to the metal sheets.
As examples of the water dispersible wax, there can be mentioned paraffin wax, microcrystalline wax, montan wax, carnauba wax, rice bran wax, Japan wax, beeswax, spermaceti, lanolin, fatty acid amide wax, Sasol wax, polyethylene wax, oxidized polyethylene wax, etc. Preferred among them are polyethylene wax and oxidized polyethylene wax.
The water dispersible wax is used after being dispersed as particles in water by a physical method, a chemical method or a combined method thereof, and water dispersions thereof are sold on the market.
It is necessary that the content of this water dispersible wax is in a range of 0.1 to 20% by weight of the total solid components including itself in the aqueous coating composition for metal sheets of the invention. When the content of the water dispersible wax in the total solid components goes beyond 20% by weight, lowering of adhesion of UV-curable top coatings and shortage of film strength occur. When the content is less than 0.1% by weight, only insufficient formability can be given to the treated metal sheet in the case of strong forming.
In the invention, the total solid components are assumed to mean the total of the resins (A), (B) and (C), the water dispersible wax (D) and the the water insoluble fine particles having a Mohs hardness of 2.5 or more (E).
When the aqueous coating composition for metal sheets of the invention containing the resin (C) alone or two or more of the resins (A), (B) and (C) or containing the water dispersible wax (D) in addition thereto further contains (E) water insoluble fine particles having a Mohs hardness of 2.5 or more in a rate of 30% by weight or less of the total solid components, coating film formed using it gives, in addition to showing good formability, good corrosion resistance, good durability with injury and good adhesion of UV-curable top coatings, better durability with injury to the metal sheet.
When the Mohs hardness is less than 2.5, the durability with injury enhancing effect cannot be shown. When the Mohs hardness is more than 7, durability with injury is better, but the surface of a metal mold used in forming after the surface treatment may sometimes be damaged depending on the material of the metal mold, and the Mohs hardness is preferably in the range of 2.5 to 7.
The water insoluble fine particles are not particularly limited so long as they satisfy the above Mohs hardness condition. As specific examples, there can be mentioned diatom earth, silica, alumina, quartz, titanium dioxide, glass, calcium carbonate, mica, iron, nickel, stainless steel, iron oxide, metal oxides, aluminum, etc.
The content of the water insoluble fine particles needs to be 30% by weight or less of the total solid components including themselves in the aqueous coating composition for metal sheets of the invention. When the content goes beyond 30% by weight, the lubricity at forming such as bending, drawing or punching of the metal sheet on which film was formed using the composition and the film strength get poor. When this content is less than 5% by weight, there may possibly occur the case where the effect to prevent the surface from being injured by external force is not enough. Thus, the content of the water insoluble fine particles is preferably 5 to 30% by weight.
The aqueous coating composition for metal sheets of the invention can, if necessary, contain various known additives such as extender pigments, colorants, electrically conducting agents, leveling agents, antifoaming agents, suspending agents or thickeners.
The aqueous coating composition for metal sheets of the invention can be prepared by dispersing the respective components in water. As the water, deionized water, etc. can be used. When the resins (A), (B) and (C) and the water dispersible wax used in the invention can be utilized as water dispersion, they can be used as such.
The aqueous coating composition for metal sheets of the invention can be applied onto various metal sheets such as cold rolled steel sheets, hot rolled steel sheets, stainless steel sheets, various zinc-series or aluminum-series plated steel sheets and aluminum or aluminum alloy sheets.
Such a metal sheet is, usually, cleaned with a solvent detergent or an alkaline or acidic aqueous detergent, and, if necessary, subjected to known undercoating treatment such as chromate treatment, iron phosphate treatment, zinc phosphate treatment, titanium phosphate treatment, zirconium phosphate treatment or organic film treatment for giving corrosion resistance, top coating adhesion, adhesion after processing etc.
Then, the composition of the invention is applied to the surface of a metal sheet. This application can be made by roll coating, dip coating, bar coating, flow coating, spray coating, brush coating, etc. After the coating, the film is dried, but this drying should be completed by volatilization of the solvent through air drying or drying by heating, and Uv curing using a photopolymerization initiator should not be carried out. When UV curing using a photopolymerization initiator is carried out, the ethylenic double bond sites contained in the aqueous coating composition of the invention are consumed in reaction and bonding, and it gets impossible to heighten adhesion of UV-curable paints, inks or adhesives as top coatings.
The amount of the dry film varies depending on the final use, but roughly 0.3 to 20 g/m2 is appropriate. When the amount is lower than 0.3 g/m2, it is impossible to attain all of good top coating adhesion, good formability, good corrosion resistance and good durability with injury. When the amount is higher than 20 g/m2, processability gets insufficient and it is not economical.
It is also possible to give functions such as designing properties, discriminating properties, corrosion resistance or adhesiveness onto the whole or part of the surface having the film formed by the aqueous coating composition for metal sheets of the invention by forming film of a UV-curable paint, ink or adhesive. As the UV-curable paint, ink or adhesive, there can be used conventional ones without particular limitation.
UV curing is a method which comprises photopolymerizing a paint, ink, adhesive, etc. and curing and drying the resutant matter in a short time by utilizing the chemical energy of ultraviolet rays. Therein, photosensitive molecules contained in the paint, ink or adhesive absorb ultraviolet rays in the presence of the photopolymerization initiator, and are polymerized and solidified. Namely, the UV-curable paint, ink or adhesive is a paint, ink or adhesive containing a photosensitive macromolecule and a photopolymerization initiator, supposed to be applied to the method.
Metal sheets having film formed by the aqueous coating composition of the invention are, in many cases, subjected to processing such as bending, deep drawing, extrusion and/or punching to be parts for household appliances, automobiles as its final uses.
The polyurethane resin having an elongatedness of dry film of 100% or more which the aqueous coating composition for metal sheets of the invention contains forms polyurethane resin film after being applied to the surface of a metal sheet as material to be coated and dried, and gives slippage, abrasion resistance, etc. of the film surface needed at forming of the metal sheet. At that time, the synthetic resin having at least one ethylenic double bond contained in the aqueous coating composition of the invention is entangled with the polyurethane resin, and contained in the fixed state in the formed polyurethane resin film. When the polyurethane resin having an elongatedness of dry film of 100% or more and having at least one ethylenic double bond is used, the above fixed state is formed as itself. Since the aqueous coating composition of the invention does not contain any photopolymerization initiator, even when it was made into film on the surface of material to be coated, the film still has ethylenic double bonds. Thus, not a few ethylenic double bonds exist on the film surface, and, if a photopolymerization initiator and ultraviolet rays and ethylenic double bonds as a partner for polymerization exist, they can cause photoreaction. When a UV-curable ink or paint containing as a main component a monomer or oligomer also having ethylenic double bonds is applied onto such surface and cured with application of ultraviolet rays, the ethylenic double bonds begin to polymerize by the action of the photopolymerization initiator contained in the UV-curable paint, ink or adhesive, and, by chain reaction, the UV-curable paint, ink or adhesive gets high molecular, including bonding with the ethylenic double bond in the undercoat film by the aqueous coating composition of the invention, whereby curing of the top coating is completed. The thus formed UV-curable top coating film has strong chemical bonds between the layers and can display good adhesion.
The water dispersible wax used in the invention is, if desired, incorporated into the aqueous coating composition for metal sheets of the invention in accordance with the purpose of use. The case to need its incorporation is, for example, a case where the resultant coated metal sheet is subjected to strong processing such as deep drawing forming, and the wax exists on or in the formed film and gives characteristics such as low deformation resistance and low surface energy, and thereby further enhances formability. Generally, when waxes exist on the surface, they make surface energy low and exist as a surface weakening layer, and therefore, top coating adhesion tends to be poor. However, since a water dispersible wax is used in the invention, the two dimensional structure of the surface of the formed film is such that the film surface formed with entanglement of the polyurethane resin having an elongatedness of dry film of 100% or more and the synthetic resin having at least one ethylenic double bond contained in the aqueous coating composition of the invention, or the film surface formed from the polyurethane resin having an elongatedness of dry film of 100% or more and having at least one ethylenic double bond is exposed from voids of dotted wax particles. Since the ethylenic double bonds exist on the exposed surface and can cause photoreaction, even in the presence of the water dispersible wax, it is possible to form strong chemical bonding with the top coating film and display good adhesion.
Further, when the water insoluble fine particles having a Mohs hardness of 2.5 or more are incorporated into the aqueous coating composition for metal sheets of the invention, as is the case with the water dispersible wax, the two dimensional structure of the surface of the formed film is such that the film surface formed with entanglement of the polyurethane resin having an elongatedness of dry film of 100% or more and the synthetic resin having at least one ethylenic double bond contained in the aqueous coating composition of the invention, or the film surface formed from the polyurethane resin having an elongatedness of dry film of 100% or more and having at least one ethylenic double bond contained therein is exposed from voids of the dotted water insoluble fine particles, and thereby good adhesion can be displayed. Futher, if a sharp-edged material body contacts with the film surface, the water insoluble fine particles can contact therewith and prevent the film from being injured.